1. Field of Invention
The present invention relates to a thin-film magnetic head having at least one of an induction-type electromagnetic transducer and a magnetoresistive element, and to a method of manufacturing such a thin-film magnetic head.
2. Description of Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of hard disk drives has increased. Such thin-film magnetic heads include composite thin-film magnetic heads that have been widely used. A composite head is made of a layered structure including a recording head having an induction magnetic transducer for writing and a reproducing head having a magnetoresistive (MR) element for reading. MR elements include an anisotropic magnetoresistive (AMR) element that utilizes the AMR effect and a giant magnetoresistive (GMR) element that utilizes the GMR effect. A reproducing head using an AMR element is called an AMR head or simply an MR head. A reproducing head using a GMR element is called a GMR head. An AMR head is used as a reproducing head where areal density is more than 1 gigabit per square inch. A GMR head is used as a reproducing head where areal density is more than 3 gigabits per square inch. It is GMR heads that have been most widely used recently.
The performance of the reproducing head is improved by replacing the AMR film with a GMR film and the like having an excellent magnetoresistive sensitivity. Alternatively, a pattern width such as the reproducing track width and the MR height, in particular, may be optimized. The MR height is the length (height) between an end of the MR element located in the air bearing surface and the other end. The air bearing surface is a surface of the thin-film magnetic head facing toward a magnetic recording medium. 
Performance improvements in a recording head are also required as the performance of a reproducing head is improved. It is required to increase the recording track density in order to increase the areal density among the performance characteristics of the recording head. To achieve this, it is required to implement a recording head of a narrow track structure wherein the width of top and bottom poles sandwiching the recording gap layer on a side of the air bearing surface is reduced down to microns or a submicron order. This width is one of the factors that determine the recording head performance. Semiconductor process techniques are utilized to implement such a structure. Another factor is a pattern width such as the throat height, in particular. The throat height is the length (height) of pole portions, that is, portions of magnetic pole layers facing each other with a recording gap layer in between, between the air-bearing-surface-side end and the other end. A reduction in throat height is desired in order to improve the recording head performance. The throat height is controlled by an amount of lapping when the air bearing surface is processed.
As thus described, it is important to fabricate well-balanced recording and reproducing heads to improve the performance of the thin-film magnetic head.
In order to implement a thin-film magnetic head that achieves high recording density, the requirements for the reproducing head include a reduction in reproducing track width, an increase in reproducing output, and a reduction in noise. The requirements for the recording head include a reduction in recording track width, an improvement in overwrite property that is a parameter indicating one of characteristics when data is written over existing data, and an improvement in nonlinear transition shift (NLTS).
Reference is now made to FIG. 16A to FIG. 22A and FIG. 16B to FIG. 22B to describe an example of a manufacturing method of a related-art thin-film magnetic head element. FIG. 16A to FIG. 22A are cross sections each orthogonal to the air bearing surface. FIG. 16B to FIG. 22B are cross sections of the pole portions each parallel to the air bearing surface.
According to the manufacturing method, as shown in FIG. 16A and FIG. 16B, an insulating layer 102 made of alumina (Al2O3), for example, having a thickness of about 5 to 10 μm, is deposited on a substrate 101 made of aluminum oxide and titanium carbide (Al2O3—TiC), for example. Next, on the insulating layer 102, a bottom shield layer 103 made of a magnetic material and having a thickness of 2 to 3 μm, for example, is formed for a reproducing head. 
Next, as shown in FIG. 17A and FIG. 17B, a shield gap film 104a made of an insulating material such as alumina and having a thickness of 10 to 20 nm, for example, is formed through sputtering, for example, on the bottom shield layer 103. Next, a shield gap film 104b made of an insulating material such as alumina and having a thickness of 100 nm, for example, is formed through sputtering, for example, on the shield gap film 104a except a region where a GMR element described later will be formed. The shield gap film 104b is provided for preventing a short circuit between the GMR element and the bottom shield layer 103.
Next, on the shield gap film 104b, a film having a thickness of 40 to 50 nm, for example, to make up the GMR element for reproduction is formed through a method such as sputtering. This film is etched with a photoresist pattern not shown as a mask to form the GMR element 105.
Next, a pair of conductive layers (that may be called leads) 106 are formed by liftoff through the use of the above-mentioned photoresist pattern. The conductive layers 106 are electrically connected to the GMR element 105. The photoresist pattern is then removed.
Next, as shown in FIG. 18A and FIG. 18B, a shield gap film 107a made of an insulating material such as alumina and having a thickness of 10 to 20 nm, for example, is formed through sputtering, for example, on the shield gap films 104a and 104b, the GMR element 105 and the conductive layers 106. The GMR element 105 is embedded in the shield gap films 104a and 107a. Next, a shield gap film 107b made of an insulating material such as alumina and having a thickness of 100 nm, for example, is formed through a method such as sputtering on the shield gap film 107a except the neighborhood of the GMR element 105.
Next, as shown in FIG. 19A and FIG. 19B, on the shield gap films 107a and 107b, a top-shield-layer-cum-bottom-pole-layer (called a top shield layer in the following description) 108 is formed. The top shield layer 108 has a thickness of about 3 μm and is made of a magnetic material and used for both the reproducing head and the recording head.
Next, as shown in FIG. 20A and FIG. 20B, a recording gap layer 109 made of an insulating film such as an alumina film and having a thickness of 0.2 μm, for example, is formed on the top shield layer 108. Next, a portion of the recording gap layer 109 located in the center of the region where a thin-film coil described later is to be formed  is etched to form a contact hole for making a magnetic path. Next, a top pole tip 110 for the recording head is formed on the recording gap layer 109 in the pole portion. The top pole tip 110 is made of a magnetic material and has a thickness of 1.0 to 1.5 μm. At the same time, a magnetic layer 119 made of a magnetic material is formed for making the magnetic path in the contact hole for making the magnetic path.
Next, the recording gap layer 109 and a part of the top shield layer 108 are etched through ion milling, using the top pole tip 110 as a mask. As shown in FIG. 20B, the structure is called a trim structure wherein the sidewalls of the top pole portion (the top pole tip 110), the recording gap layer 109, and a part of the top shield layer 108 are formed vertically in a self-aligned manner.
Next, an insulating layer 111 of alumina, for example, having a thickness of about 3 μm is formed over the entire surface. The insulating layer 111 is polished to the surfaces of the top pole tip 110 and the magnetic layer 119 and flattened.
Next, as shown in FIG. 21A and FIG. 21B, on the flattened insulating layer 111 a first layer 112 of the thin-film coil is made for the induction-type recording head. The first layer 112 of the coil is made of copper (Cu), for example. Next, a photoresist layer 113 is formed into a specific shape on the insulating layer 111 and the first layer 112 of the coil. Next, a second layer 114 of the thin-film coil is formed on the photoresist layer 113. Next, a photoresist layer 115 is formed into a specific shape on the photoresist layer 113 and the second layer 114 of the coil.
Next, as shown in FIG. 22A and FIG. 22B, a top pole layer 116 for the recording head is formed on the top pole tip 110, the photoresist layers 113 and 115 and the magnetic layer 119. The top pole layer 116 is made of a magnetic material such as Permalloy. Next, an overcoat layer 117 of alumina, for example, is formed to cover the top pole layer 116. Finally, machine processing of the slider including the forgoing layers is performed to form the air bearing surface 118 of the thin-film magnetic head including the recording head and the reproducing head. The thin-film magnetic head is thus completed.
FIG. 23 is a top view of the thin-film magnetic head shown in FIG. 22A and FIG. 22B. The overcoat layer 117 and the other insulating layers and film are omitted in FIG. 27.
In order to improve the performance characteristics of a hard disk drive, such as areal recording density, in particular, a method of increasing linear recording  density and a method of increasing track density may be taken. To design a high-performance hard disk drive, specific measures taken for implementing the recording head, the reproducing head or the thin-film magnetic head as a whole depend on whether linear recording density or track density is emphasized. That is, if priority is given to track density, a reduction in track width is required for both recording head and reproducing head, for example. If priority is given to linear recording density, it is required for the reproducing head to improve the reproducing output and to reduce the half width of the reproducing output. Moreover, it is required to reduce the distance between the hard disk platter and the slider (hereinafter called a magnetic space). To achieve areal density of 20 to 30 gigabits per square inch, a magnetic space of 15 to 25 nm, for example, is required.
Consideration will now be given to the measures taken when priority is given to linear recording density. Among the factors that contribute to improvements in linear recording density, a reduction in magnetic space is achieved by reducing the amount of floating of the slider. The amount of floating of the slider depends mainly on the design, processing method, lapping method and so on of the slider.
Among the factors that contribute to improvements in linear recording density, an improvement in reproducing output is achieved mainly by replacing the AMR film with a GMR film and the like having an excellent magnetoresistive sensitivity. It is known that another factor, that is, a reduction in half width of the reading output, is achieved by reducing the distance between the bottom shield layer and the top shield layer (hereinafter called the shield gap length). It is possible to control the shield gap length it the steps of manufacturing the thin-film magnetic head.
The problems arising when the shield gap length is reduced will now be described. To implement areal recording density of about 10 gigabits per square inch, an appropriate shield gap length is 0.11 to 0.14 μm (110 to 140 nm). However, a shield gap length of 0.07 to 0.09 μm (70 to 90 nm) is required for implementing areal recording density of 30 to 40 gigabits per square inch.
It is difficult to reduce the thickness of the MR element since this thickness is determined by factors such as the reading output required. Therefore, in order to reduce the shield gap length, it is required to reduce the thickness of the shield gap film provided between the MR element and the bottom shield layer, and the thickness of the shield gap film provided between the MR element and the top shield layer. 
A case is assumed wherein a shield gap length of 60 to 70 nm is required to implement areal recording density of 40 gigabits per square inch. In this case, if the thickness of the MR element is 40 nm, the thickness of the shield gap films each of which is provided between the MR element and the bottom shield layer and between the MR element and the top shield layer, respectively, is required to be 10 to 15 nm.
In prior art the shield gap film is made of an alumina film formed through sputtering performed in a plasma atmosphere through the use of an apparatus such as a radio frequency (RF) sputtering apparatus or an electron cyclotron resonance (ECR) sputtering apparatus.
However, a reduction in the thickness of the prior-art shield gap film formed through sputtering is limited to about 20 nm. That is, if the thickness of the prior-art shield gap film is smaller than 20 nm, the insulation strength is 5 to 7 volts or smaller so that static damage is likely to occur. If the thickness of the prior-art shield gap film is reduced down to about 10 to 15 nm, not only the insulation strength is made smaller but also pinholes are likely to occur. If static damage is done to the shield gap film or pinholes are made in the shield gap film, a short circuit is developed between the MR element and the bottom shield layer or the top shield layer. As a result, the reading output signal carries noise, and it is impossible to obtain a proper reading output signal in some cases.
In addition, the prior-art shield gap film exhibits bad step coverage. Therefore, pinholes or faulty insulation frequently occurs in portions having projections and depressions, in particular, such as the neighborhood of the pattern edge of the MR element or the leads connected to the MR element.
As thus described, it is difficult in prior art to form the shield gap film that is thin and exhibits high qualities, that is, closely packed and has an even thickness, greater insulation strength and excellent step coverage. Therefore, it is difficult to reduce the shield gap length of the prior art thin-film magnetic head, and to reduce the half width of the reading output and to improve the recording density. In addition, since it is difficult in prior art to form a high-quality and thin shield gap film, the yield of thin-film magnetic heads for high density recording is low.
Although the problems arising when the shield gap film is formed have been described so far, similar problems are found in formation of layers such as the recording gap layer, an insulating film of a thin-film magnetic head wherein the recording  head and the reproducing head are isolated from each other by the insulating film, or an insulating layer for isolating turns of the coil.